Optically-pumped solid-state lasers have been useful sources of coherent radiation for more than 20 years. In recent years, improvements in laser performance have resulted from the fabrication of semiconductor quantum-well structures. In a quantum-well structure, thin atomic layers of smaller energy bandgap semiconductor material, for example GaAs, are sandwiched between thin layers of wider bandgap material, for example Al.sub.x Ga.sub.(1-x) As, to form potential wells in the conduction and valence bands. Such wells restrict or limit carrier/electron movement to two dimensions. Quantum-well heterostructures generally exhibit more efficient luminescence intensities than bulk crystal heterostructures and therefore, have been incorporated into the active region of semiconductor laser devices.
In current solid-state microchip lasers, the laser cavity geometry determines the volume of a laser mode propagating therein. A typical microchip laser cavity comprises a gain medium disposed between a pair of curved mirrors. Incoherent pump energy, for example laser diode output, is directed into the cavity using various techniques. Certain pumping techniques are more efficient than others.
Laser efficiency is the proportion of pump energy which is converted to coherent output energy. Pump energy directed into the cavity that is not converted to output energy is emitted as wasted incoherent energy or converted into heat. The more pump energy that is wasted, the lower the efficiency of the laser.
In the past, solid-state lasers were side-pumped in a transverse direction with incoherent diodes along the length of the gain medium. Side-pumped lasers are relatively inefficient, because the entire gain medium volume is pumped, with laser gain occurring only in a small central volume of the gain medium. The pump energy is dispersed throughout the gain medium, rather than being concentrated within the volume of the resonant laser mode. This results in inefficient energy conversion.
Recently, end-pumped solid-state lasers have improved conversion efficiency. In end-pumped lasers such as those described in U.S. Pat. No. 4,710,940, incoherent pump energy is longitudinally focused with optics through the entrance and/or exit mirrors into the central portion of the gain medium within the diameter of the resonating laser mode. This technique is thus limited in that the laser mode volume is predetermined by the geometry of the laser cavity, and therefore, the diameter of the pump beam must be matched to the predicted laser mode diameter for efficient operation.